Electronic screening for image reproduction is well established in the art. According to a well known technique described in U.S. Pat. No. 4,456,924 of the present assignee, for each screened dot, a multiplicity of coordinates of a laser plotter are translated into screen-cell coordinates. A corresponding cell memory is preloaded with threshold values, defining a cell memory matrix. Input digitized scanned density values of an image, such as a color separation, are compared with the threshold values, cell by cell. The results provide an on/off control input for a laser plotter.
U.S. Pat. No. 4,825,298 to Ikuta and Murai describes a technique for generating a screened reproduction of an image in which the density distribution of a given screen dot is expressed in three dimensions, wherein the area of the screen dot is expressed along X and Y axes and the density is expressed along a Z axis perpendicular thereto. A film coordinate generator generates film coordinates (u,v), corresponding to the position of an exposure beam on a recording film which position is detected by encoders. The film coordinates are in turn supplied to a screen coordinate generator to be converted into virtual screen coordinates (x,y). A beam control signal generator receives the coordinates (x,y) and an image signal corresponding to the position of the exposure beam to output a beam control signal indicting lighting of the exposure beam when one of the coordinates (x,y) is between upper and lower limit values, corresponding to the same, which are previously determined for each combination of the other of the coordinates (x,y) and the density value of the image signal.
The technique of U.S. Pat. No. 4,825,298 to Ikuta and Murai has the disadvantage that its memory requirements are high.
U.K. Published Patent Application 2,157,119A to Ikuta describes apparatus which operates similarly to the technique of U.S. Pat. No. 4,456,924 but does not employ a matrix memory. Instead, the threshold function is calculated on the fly in real time or near real time. This apparatus is limited to relatively simple dot configurations.
In U.S. Pat. No. 4,918,622 there is described an electronic graphic arts screener in which a three-dimensional memory array is employed for screen dot generation.
Conventional techniques for screen dot generation, exemplified by that described in U.S. Pat. No. 4,456,924 represent each input density level by a screen dot of a corresponding number of dot elements. The difference between a relatively lower input density and a relatively higher input density is indicated by adding predetermined additional dot elements at predetermined locations to those dot elements at predetermined locations which represent the lower input density. Thus the set of dot elements which makes up a relatively higher input density dot always includes the subset of the dot elements which makes up a relatively lower input density dot.
The foregoing arrangement does not always provide a best fit to a chosen screen dot shape for a given input density.